[Bug 230888] Missing 64 bit atomic functions for i386 (libatomic)

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230888

--- Comment #12 from Dimitry Andric <dim at FreeBSD.org> ---
This issue has now come up again with the import of llvm 9.0.0 (which is
ongoing).  Upstream has committed the following change,
https://reviews.llvm.org/rL356631 ("[X86] Add CMPXCHG8B feature flag. Set it
for all CPUs except i386/i486 including 'generic'. Disable use of CMPXCHG8B
when this flag isn't set")

Before that change, clang actually had a bug, in the sense that it *did*
sometimes generate cmpxchg8b instructions on i486, which is still our default
target CPU.  This saved us from most cases where otherwise __atomic_xxx
function calls would be inserted.  However, now this bug has been fixed, so it
will always use function calls for atomic 64 bit operations.

Upstream, I have had to report that it cannot even do the unit tests for
compiler-rt 9.0.0 anymore, since that already runs into link errors about
__atomic_xxx functions: https://bugs.llvm.org/show_bug.cgi?id=42892

Realistically, the best way forward in the short term is to raise the default
target CPU for FreeBSD to i586 or even i686, so cmpxchg8b is supported. 
Specifically, because I do not think it is very realistic to run modern FreeBSD
on real i486-class hardware.  Not to mention that that is basically museum
class hardware now. :-)

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